This invention relates to a data communication network and method of communication therein, and has particular application to a local area network which provides communication between a plurality of data processing stations located within a moderately-sized, geographical area, e.g. a multi-story office building.
In local area networks, a passive communication medium such as a coaxial, twisted pair or fibre optic cable provides a common serial data channel to interconnect the data stations. Each data station may, for example, comprise a micro- or minicomputer, a memory disc system, a printer, an intelligent or non-intelligent video display unit or an interface to a public communications network. A transmitting station transmits frames or packets of data of predetermined format in accordance with a standarized protocol over the data channel. The station for which the transmission is intended recognizes the address in a data frame and receives the data. A network architecture including higher levels of control software is provided for efficient control of the network.
Difficulties arise in scheduling transmissions in busy periods when more than one station may wish to transmit at the same time.
In European patent application No. 000,3849 there is disclosed a time-division, multiplexing scheme whereby the risk of two or more stations transmitting at the same time and of the transmissions colliding is avoided. However, such a scheme which operates solely by allocating fixed time intervals for transmission for each station is not efficient when communication traffic occurs in bursts, since a large part of the time available for transmission may be wasted.
One well-known method of scheduling transmissions designed specifically for traffic occurring in bursts is used in the Ethernet system and disclosed, for example, in U.S. Pat. No. 4,063,220. In this system, when a station wishes to transmit, the presence of other transmissions on the data channel is detected, and transmission is delayed until no other transmissions are detected. Once a transmission is initiated, if interference or collision is detected with a transmission initiated at about the same time by another station, transmission at each station is aborted. A collision enforcement mechanism temporarily jams the channel to ensure that all stations participating in the collision are aware of the collision and will abort. A random number generator is employed at each station to determine an interval of time at the completion of which the next attempted transmission takes place so as to resolve the contention for the channel.
The Ethernet system has certain disadvantages since in busy periods to avoid blocking of the channel it is necessary, after a certain number of attempts at retransmission, to reschedule the transmission at a higher level of control in the network architecture. A further disadvantage is the requirement for reformatting the data package whenever retransmission is attempted. In addition the channel is jammed for a relatively long period whenever a collision occurs.
Another approach which has been employed for scheduling transmissions is the assignment of a transmission priority to each data station, as disclosed for example in International patent application No. WO 80/01426. In the event of contention for the channel by more than one data station, the contention is resolved in favor of the station with the higher, assigned priority. However, a disadvantage of such a priority scheme is that in busy periods stations with a low priority may not gain access to the channel.